1. Field of the Invention
The present invention relates generally to a heat exchanger, and more specifically to a ceramic heat exchanger for use with a high temperature fluid.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine is one of the most efficient engines for converting the combustion of a fuel into mechanical work. Small gas turbine engines have been proposed for use in vehicles because of their efficiency. However, without the use of a heat exchanger (also referred to as a regenerator or a recuperator when used in a gas turbine engine) to form a combined cycle power plant, the small gas turbine engine is less efficient than a diesel engine. The diesel engine, although a high efficient engine, is also a high polluting engine. In order to replace the diesel engine with a small gas turbine engine, a workable regenerator is required to raise the efficiency to that above the diesel engine.
Heat exchangers (regenerators) are well known in the art for use with gas turbine engines in order to produce a combined cycle power plant. In a combined cycle power plant, the exhaust heat from the turbine outlet is used to preheat the air fed into the compressor in order to increase the efficiency of the engine. in some cases, the compressor outlet air is heated with the regenerator prior to entering the combustor. Prior art heat exchangers are made of materials that have high heat transfer coefficients in order to produce high heat transfer amounts. Copper is one well known material for use in heat exchangers. However, copper has a relatively low melting temperature when compared to the outlet temperatures of a gas turbine engine.
One reason why heat exchangers have not become practable for use with smaller gas turbine engines is because of the high thermal stresses developed from combining the high temperature turbine exhaust gas with the low temperature inlet air for the compressor in the heat exchanger. These high thermal stresses that develop produce cracks in the tubes where the heat exchanger has been brazed or welded in its construction. The tubes that are used are typically brazed or welded to the end plates and the baffles. The high temperature difference between the hot fluid and the cooler fluid will produce thermal stresses that lead to cracks between the brazed or welded tube and the baffle or end plate. The cracks will produce unacceptable leakage and make the heat exchanger useless.
Another reason why heat exchangers have not become practable for use in smaller gas turbine engines is that they tend to be much larger than the engine in order to provide for the required increase in efficiency of the combined cycle power plant. Also, the passages formed within the heat exchanger can be complex and thus very costly to manufacture. The surface area for the heat transfer between the hot fluid and the cold fluid must be as large as possible in order to improve the efficiency of the heat exchanger and therefore gas turbine engine. Providing for long passages through the heat exchanger increases the efficiency. In some cases, the cost of the heat exchanger would be much more than the cost of the small gas turbine engine.